Nothing as dramatic as some of the others, but I had the attitude indicator fail on the last lesson before my skills test - the gyros slowly spun down over 2 or 3 minutes. Told the instructor we would be returning to land, and he said I'd made the right decision but we'd carry on as it was a crystal clear day. 15 minutes later the instrument came back to life again and appeared normal - cue very smug instructor telling me he knew we had done the right thing continuing - although I refused to use the gyro instruments for the rest of the flight and used that funny compassy thingy that's stuck on the dashboard while actually looking at the horizon.
Never found out what the problem was - the heading indicator seemed unaffected and we had 5 inches of suction so there was no major vacuum system problem.
Funny thing was, a couple of days earlier we'd been doing some real IMC practice, making climbing and decending turns through a cloud layer - in the same aircraft. Makes you wonder if you really want to fly IMC in a light single, but that's another thread...